When is white not white? When it's 0% black, of course! Here's how a client nearly got 15,000 CDs printed with his primary sponsor's logo not visible.

Tip 1: If you must proof a press PDF electronically instead of using a matchprint, use Adobe Acrobat or Reader and turn on Overprint Preview, (it's quite a good idea to turn off Use Local Fonts as well). Never trust OSX's Preview, as it won't show overprints. Oh, and set Illustrator and InDesign to view overprints as well, while you're at it.

Tip 2: don't convert something from black to white by reducing the black ink from 100% to 0: make it 100% white instead.

Tip 3: always check your trapping: the best way is to generate colour-separated PDFs as well as composite ones.

Here's what happened, and I only mention it because this designer was the third chap in the last few weeks to make a similar mistake.

The design included a white Illustrator line art logo on a transparent background; the logo had been supplied by the Arts Council to the designer in black, with overprint fill ticked on; the designer wanted a white logo, so he just wanged the ink slider all the way to the left (he said it seemed a good idea at the time). He didn't notice that overprint fill was on, and didn't think anything was wrong because he could see the logo in Illustrator, Acrobat, and OSX Preview; I of course didn't know it was meant to be there, as it wasn't visible in my Acrobat because I use overprint preview so I can see how jobs will look on press. I only noticed it when I did the matchprint for the press operator: the RIP on my proofer decided it had better print the logo anyway, and I just happened to spot it, thank Heavens!

My Acrobat preflight will jump and down and start shouting if it encounters a white object set to overprint, but it didn't pick up this one because the logo wasn't white, technically-speaking: it was 0% black. This designer had a lucky escape, because the press would certainly not have printed that logo!